Network main admin site suddenly not available, front-end & sub-admin pages still alive

Network main admin site suddenly not available, front-end & sub-admin pages still alive. While I was working in one of the networked based sites suddenly the networked admin environment went offline (domain.xx/wp-admin/). Out of the blue it`s not available anymore. The backend sub-sites are still alive (domain.xx/subsite/wp-admin/). All front-end sites are still alive and available without any error.

The browser error message: The connection was reset. The connection to the server was reset while loading the page.

Before the ‘crash’ I did not installed any new plugins nor did some changes in the configuration. I don`t think it has to do something with update to WordPress 3.9. I did not update my subsites to 3.9 and it has worked fine for a few days.

Already tried to do a database repair but that didn`t fix the problem.

Installed different WPMU plugins as base MarketPress. Because I cannot get in to the main networked admin panel I cannot see all the installed plugin`s...